In a single-wavelength optical transmission system transmitting data with single wavelength light, there has been put into actual use a system having duplicated (redundant) transmission lines, i.e. the main transmission line and the backup transmission line, for a station apparatus of a single (non-duplicated) configuration, thereby providing fault tolerance ability to the transmission line. Provision of fault tolerance ability with multiple configuration for each station apparatus may be considered, but this is not adopted practically because cost increases.
FIG. 18 is a block diagram illustrating an overview of the single-wavelength optical transmission system having duplicated transmission lines. In this single-wavelength optical transmission system, terminal stations 101, 103, and a relay station 102 are provided as station apparatuses. In some cases, more than two relay stations, instead of one, may be configured in the system.
The transmission lines provided between the neighboring station apparatuses are duplicated. Between each station apparatus, main transmission lines F01, F02, R01 and R02 and backup transmission lines F11, F12, R11 and R12, which are made of light fiber, are provided. The main transmission lines are used normally, while the backup transmission lines are used when a failure (for example, fiber break and deterioration, etc. caused by secular change, etc.) occurs on the main transmission line.
Each station apparatus includes optical amplifiers (AMP. For example, Erbium-doped fiber amplifiers: EDFA) 201-206, so as to amplify input optical signals. Further, each station apparatus includes optical switches (OSW) 301-308 for transmitting input optical signals to either the main transmission line or the backup transmission line. The optical switches also receive the optical signals from either the main transmission line or the backup transmission line. Since a section between the stations is termed as span, in some cases, optical switches 301-308 may be termed as span switches.
These optical switches are switched to the main transmission line side when there is no failure (normal state). In the event that a failure occurs in the main transmission line, the corresponding optical switches are switched over to the backup transmission line side. For example, in the normal state, optical switch 301 is in the state switched to the main transmission line F01 side, and thereby an optical signal input from optical amplifier 201 is output to the main transmission line F01 side. In the event that a failure occurs in the main transmission line F01, optical switch 301 is switched over to the backup transmission line F11 side, and the optical signal from optical amplifier 201 is output to the backup transmission line F11.
Meanwhile, as a communication system enabling high-speed transmission of a large amount of data, a wavelength division multiplex (WDM) optical communication system has received wide attention. This WDM optical communication system multiplexes optical signals into different wavelength signals, which transmits on one optical fiber.
In such a WDM optical communication system, implementation of a fault tolerant system is also desired. Similar to the single-wavelength optical communication system shown in FIG. 18, a system has been put into practical use, such that optical fiber transmission lines between the station apparatuses have duplicated (redundant) configurations, consisting of the main transmission line and the backup transmission line, in contrast to the station apparatuses having single (non-duplicated) configurations, and that these transmission lines are switched by the optical switches.
However, most of the conventional WDM optical communication systems have the same number of optical switches as the number of multiplexed wavelengths, and each optical switch switches the optical signal on a wavelength-by-wavelength basis. Accordingly, in the conventional WDM optical communication system, with the increase of the number of the multiplexed wavelengths (for example, 160 waves, 320 waves, etc.), the cost has become increased because the provision of the optical switches with the same number as the number of the wavelengths is necessary.
Accordingly, there has been desired the WDM optical communication system in which the WDM multiplexed optical signals are switched collectively by a single optical switch.
However, in order to put into actual use such a WDM optical communication system using an optical, switch for collectively switching optical signals, there are some problems to be solved, differently from the optical communication system using the conventional optical switch switching the optical signals on a wavelength-by-wavelength basis.
The first problem is how to compensate a transmission loss difference caused by the transmission line (optical switch) switchover.
When the transmission line is switched over from the main transmission line to the backup transmission line as a result of the switchover of the optical switch, an attenuation factor (loss) of the transmission line is varied, producing a varied signal level to be input to an optical amplifier in the succeeding stage. When the input signal level is varied, the level of amplified spontaneous emission (ASE) light (noise component) generated from the optical amplifier also varies. This produces an undesirable increase of possibility that the further succeeding optical amplifier becomes unable to amplify the signal component (communication signal component) included in the input signal to a predetermined level.
In order to solve such a problem, conventionally, variable optical attenuators have been provided on both, or either of, the main transmission line and the backup transmission line. By adjusting the attenuation factor of any variable optical attenuator, the optical signal level input from the main transmission line to the optical amplifier can be set substantially equal to the optical signal level input from the backup transmission line to the optical amplifier.
This method, however, has limit to cope with a variety of network structures. For example, this method is not applicable when the lengths of the main transmission line and the backup transmission line largely differ, resulting in that the difference of the attenuation factors between both transmission lines exceeds beyond an adjustable range of the variable optical attenuator. For this reason, it becomes necessary to introduce another method, so that each optical amplifier can amplify each signal component to a predetermined level without adjusting the attenuation factor.
The second problem is how to prevent a receiving quality deterioration of the optical signal caused by the transmission line (optical switch) switchover.
In some cases, an aspect of tilt produced in the optical signal is varied when a wavelength dependent loss (WDL) of the transmission line or the flatness in the gain of the intervened optical amplifier is varied after the switchover of the transmission line from the main transmission line to the backup transmission line, or vice versa, by use of the optical switches. This may produce an error in a particular wavelength signal at a receiving end, because of non-uniformity of the signal quality being produced among the respective wavelengths.
To cope with this problem, it is necessary to prevent the signal quality of each wavelength from deterioration even when the switchover is performed, by compensating the difference in the tilt.
The third problem is how to prevent generation of a light surge.
The light surge is generated by a burst emission of the energy accumulated in the optical amplifier (such as EDFA) during a light interception period of no optical signal input into the optical amplifier, when the optical signal input is resumed after the light interception.
In the WDM optical communication system, the light interception is produced when an optical signal is intercepted in the optical switch during the switchover time (for example, ten and a few milliseconds), and accordingly no optical signal is input to the amplifier. Also, the light surge is generated when the optical signal is input again to the optical amplifier through the optical switch, after the switchover completion of the optical switch.
Especially in the WDM optical communication system, since the optical signals are multiplexed, output power (output electric power and output level) from the optical amplifier becomes larger, in proportion as the increased number of wavelengths. Because the light surge is superposed onto this output level, the resultant output level becomes remarkably high, which may cause damage, such as melting of the optical fiber transmission line.
To prevent generation of such a light surge, one method may be considered so as to adjust the light interception period to a time sufficient for emitting the residual energy accumulated in the optical amplifier. However, this method is not preferable because a longer signal interception period than a tolerable time is produced by the switchover of the optical switch longer.
Accordingly, a method for preventing generation of the light surge, enabling the light interception period within the tolerable signal interception period, is required.